Salt on Mars May Have Melted Red Planet's Ice

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Water could have flowed on the surface of Mars with the help of
salts in the Martian soil that can melt ice, just as salts on
Earth can be used to melt ice on slippery winter roads and
sidewalks, researchers say.

These findings suggest "the shallow subsurface of Mars could be
habitable," study co-author Nilton Rennó, a planetary and
atmospheric scientist at the University of Michigan at Ann Arbor,
told Live Science.

Liquid water is thought to be an essential ingredient for life as
we know it, and there is life virtually wherever there is liquid
water on Earth. As such, when researchers search for life
elsewhere in the universe, they often look for places that could
harbor liquid water. [ 7
Most Mars-like Places on Earth ]

Observations of the Red Planet suggest
water once flowed on the surface of Mars, such as the gullies
found running down crater rims. In September 2013, NASA's Mars
rover Curiosity discovered water locked up in the planet's
surface soil. The robotic explorer found that each cubic foot of
Martian dirt (0.03 cubic meters) contains about 2 percent water.

"By studying the formation of liquid water on Mars, we can learn
about possibilities of life outside Earth and look for resources
for future missions," lead study author Erik Fischer, a planetary
scientist at the University of Michigan at Ann Arbor,
said in a statement.

It has long been a mystery how liquid water could form on the
surface of Mars. The Red Planet's atmosphere is both cold and
thin, which means water usually freezes on Mars, and is not found
in either liquid or vapor form.

However, in 2008, Rennó noticed strange globules in photos sent
back by NASA's Phoenix Mars Lander, which operated on the Red
Planet from May 2008 to November 2008. Over several weeks, the
globules seemed to grow and coalesce.

Rennó suggested these mysterious formations were visible because
Phoenix's landing thrusters likely blasted away Martian topsoil,
exposing and melting the ice underneath it. Salts on the planet's
surface then kept the droplets liquid, because saltwater freezes
at lower temperatures than ordinary water does. But the salts on
Mars are different from common table salt — technically known as
sodium chloride. Martian salts likely include calcium
perchlorate, a mixture of calcium, chlorine and oxygen that is
found in arid places on Earth, such as the Atacama Desert in
Chile.

Still, while salt can help keep water liquid, it is uncertain
whether salt can explain the presence of vast amounts of liquid
water that research suggests once flowed across Mars. Although
salt can help liquid water form by sucking water vapor from the
air, this process, called deliquescence, is slow, and cannot
explain the volumes of
water that once may have covered Mars.

To shed light on the potential role that salt plays on Mars,
Fischer, Rennó and their colleagues recreated the Phoenix landing
site conditions in their lab in metal cylinders measuring 2 feet
(0.6 meters) high and 5 feet (1.5 m) long. The scientists
mimicked temperatures in the late Martian spring and early
summer, which typically range from minus 185 to minus 5 degrees
Fahrenheit (minus 120 to minus 21 degrees Celsius), as well as
atmospheric pressure, which is about 1 percent that of Earth's.
For most experiments, humidity was set to 100 percent.

The researchers found that in experiments involving only humid
air and layers of calcium perchlorate that were only 0.04 inches
(1 millimeter) thick, no liquid water formed, even after more
than three hours. This suggested deliquescence was not occurring,
and is probably not a significant process on Mars, the
researchers said. [ 7
Theories on the Origin of Life ]

However, when the scientists placed calcium perchlorate or salty
soil directly on a 0.1-inch-thick (3 mm) ice layer, drops of
liquid water formed within minutes when the chambers reached
minus 100 degrees F (minus 73 C).

These findings reveal that Martian salts can help liquid water
form, but only when the salts touch ice. "It takes ice to make
liquid water on Mars," Rennó said.

The researchers found that the conditions that support
salty water on the Red Planet can last for a few hours per
day during late spring and early summer at the polar regions of
Mars. "They probably can last weeks per year or even longer in
the shallow subsurface at mid-latitudes," Rennó said. "We are
talking about the formation of small amounts of liquid water like
droplets, not rivers and lakes."

Rennó suggested that saltwater would not have to stay liquid
indefinitely for it to support microbial life on Mars now, or in
the past.
Antarctic saltwater and lattices of brine-filled ice-combs
have been found to harbor
microbial organisms on Earth.

The most important implication of these findings "is that when
looking for signs of liquid water on Mars, it would be good to
look at conditions that we have shown in our experiments,"
Fischer told Live Science.

"Rovers sent to equatorial and low latitudes are not likely to
find liquid water because water ice is not stable there on the
surface and in the shallow subsurface," Fischer explained.
Mid-latitudes and polar regions are the best candidates for
liquid water, because both ice and warm enough temperatures are
present, "and salts have already been detected by the Phoenix
lander," he added.

Fischer, Rennó and their colleagues, Germán Martínez and Harvey
Elliott, detailed their findings in a paper accepted for
publication June 18 in the journal Geophysical Research Letters.